The present invention relates to an ink jet printing apparatus for a computer, facsimile transceiver or the like. The apparatus is constructed to print alphanumeric characters, graphic designs or any other type of desired pattern in response to applied electric signals.
A printing apparatus of this type comprises an ink ejection head which is moved relative to a printing surface such as a sheet of paper in an orthogonal scanning pattern. Electrical signals in the form of pulses are applied to the head to cause ink ejection from the head onto the paper. The signals correspond to the pattern to be printed. Each pulse causes ejection of one drop of ink onto the paper to print one dot.
In order to print various shades of gray rather than just black and white, the head is constructed in such a manner that the amount of ink ejected and therefore the density of the printed dot varies in accordance with the magnitude of the corresponding applied pulse. The larger the magnitude of the pulse the greater the amount of ink ejected.
However, due to the basic design of the ink ejection head the velocity of the ejected ink also increases with the applied signal magnitude. In other words, the ink leaves the head at a greater initial velocity for high signal levels than for low signal levels. At the maximum signal level, the high velocity ink will be incident at a second point on the paper which is downstream of the first point in the direction of movement of the head. The reason is that the low velocity ink takes longer to reach the paper than the high velocity ink and that the ink has a transverse velocity component in addition to a normal velocity component. The normal velocity component increases with the magnitude of the applied pulse and is due to the force of ejection of the ink from the head. The transverse velocity component is substantially constant and is due to the movement of the head. The displacement of the printed dot from the ejection position of the head is substantially equal to the time of flight of the ink from the head to the paper multiplied by the velocity of movement of the head.
This effect is undesirable, especially in a facsimile transceiver or the like since it produces a distorted printed pattern. The position of a particular dot on the paper is a function of the density of the dot whereas the position of the dot should be the same regardless of the density of the dot.
The distortion caused by this effect is even more undesirable in a color ink jet printer which comprises three ink ejection heads for ejecting ink of three primary colors. A dot of the desired color and density is produced by superimposing three dots of the primary colors from the respective ink jet heads. However, since the magnitudes of the pulses applied to the ink jet heads and thereby the velocities of ink ejection will be different for the three respective colors, the primary color dots will not be superimposed on each other but will be displaced from each other in the direction of movement of the heads. The result is not only positional distortion but also color distortion.